Code Division Multiple Access (CDMA) communication systems may be used to provide wireless communication between a base station and one or more subscriber units. The base station is typically a computer controlled set of switching transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units. The base station antenna is also responsible for receiving reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse link signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a wireless cellular modem.
The most common type of antenna used to transmit and receive signals at a mobile subscriber unit is an omni-directional monopole antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. The transceiver receives reverse link signals to be transmitted from circuitry within the subscriber unit and modulates the signals onto the antenna element at a specified frequency assigned to that subscriber unit. Forward link signals received by the antenna element at a specified frequency are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit. In CDMA cellular systems, multiple mobile subscriber units may transmit and receive signals on the same frequency and use coding algorithms to detect signaling information intended for individual subscriber units on a per unit basis.
The transmitted signal sent from a monopole antenna is omnidirectional in nature. That is, the signal is sent with the same signal strength in all directions in a generally horizontal plane. Reception of signals with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal on one direction versus detection of the same or a different signal coming from another direction.
Various problems are inherent in prior art antennas used on mobile subscriber units in wireless communications systems. Typically, an antenna array with scanning capabilities consists of a number of antenna elements located on top of a ground plane. For the subscriber unit to satisfy portability requirements, the ground plane must be physically small. For example, in cellular communication applications, the ground plane is typically smaller than the wavelength of the transmitted and received signals. Because of the interaction between the small ground plane and the antenna elements, which are typically monopole elements, the peak strength of the beam formed by the array is elevated above the horizon, for example, by about 30xc2x0, even though the beam itself is directed along the horizon. Correspondingly the strength of the beam along the horizon is about 3 db less than the peak strength. Generally, the subscriber units are located at large distances from the base stations such that the angle of incidence between the subscriber unit and the base station is approximately zero. The ground plane would have to be significantly larger than the wavelength of the transmitted/received signals to be able to bring the peak beam down towards the horizon. For example, in an 800 Mhz system, the ground plane would have to be significantly larger than 14 inches in diameter, and in a PCS system operating at about 1900 Mhz, the ground plane would have to be significantly larger than about 6.5 inches in diameter. Ground planes with such large sizes would prohibit using the subscriber unit as a portable device. It is desirable, therefore, to direct the peak strength of the beam along the horizon with antenna elements mounted on a small ground plane so that the subscriber unit is mobile. Further, it is desirable to produce antenna elements with these beam directing features using low-cost mass production techniques.
The present invention greatly reduces problems encountered by the aforementioned prior art antenna systems. The present invention provides an inexpensive monopole antenna for use with a mobile subscriber unit in a wireless same frequency network communications system, such as CDMA cellular communication networks. The antenna includes a radiating element located near a feed point to minimize transmission delay from the feed point to the element, and a ground patch located above the element to to force the beam peak down from about 30xc2x0 to about 10xc2x0. The antenna is fabricated with printed circuit board (PCB) photo-etching techniques for precise control of the printed structure.
In one aspect of the invention, the monopole antenna includes a planar substrate made of dielectric material. A conductive planar element is layered on one side of the substrate, and a conductive planar ground patch is layered on the other side of the substrate. The conductive planar element is located in a lower region of the substrate, while the location of the conductive planar ground patch is offset from the conductive planar element in an upper region of the substrate, that is, the ground patch is stacked above the conductive planar element. The conductive planar element includes a feed point which is typically connected to a transmission line for transmitting signals to and receiving signals from the antenna. A strip is connected to the conductive planar ground patch and extends from the patch to a bottom edge of the substrate for coupling the ground patch to a ground plane upon which the antenna is mounted.
In this arrangement, the conductive planar element acts as a monopole element to transmit and receive signals. The ground patch, being positioned above the monopole element, forces the beam transmitted from the antenna to be directed along the horizon.
Embodiments of this aspect can include one or more of the following features. Both the conductive planar element and the conductive planar ground patch are shaped as square to maximize the bandwidth of the antenna. Alternatively, the planar element can have a T-shape with the feed point being located at the bottom of the T-shaped element. Further, the planar element can include downward extensions connected on either side of horizontal portion of the T-shaped element. Or the conductive planar element can include two portions, the first portion being a vertical strip, and the second portion having two arms, each connected to a top end of the first portion and tapering down and away from the vertical strip.
The dielectric material is made from, for example, common PCB materials, such as polystyrene or Teflon. The conductive planar element and the conductive planar ground patch are typically made from copper.
In one embodiment of this invention, the conductive planar element is connected to a phase shifter. The phase shifter is independently adjustable to affect the phase of a respective signal transmitted from the dipole antenna. Alternatively, the planar element is connected to a delay line. The antenna can be connected to a variable or lumped impedance element and/or a switch. Ideally, the peak strength of the directed beam rises no more than about 10xc2x0 above the horizon.